The present invention concerns a CDMA (Code Division Multiple Access) method with increased capacity. In practice the invention concerns a method of transmitting a plurality of signals by radio using a spectrum spreading technique in which each symbol to be transmitted is encoded by orthogonal sequences of bits (O sequences) or random or pseudo-random sequences of bits (PN (pseudo-noise) sequences). The decoding carried out at the receiving end is a function of the encoding. The aim of the invention is to increase the number of messages that can be transmitted using a method of the above kind, in particular beyond its known theoretical limit.
Generally speaking, in the case of transmitting a signal for a single user using a given modulation technique (amplitude, frequency, or phase modulation, or a hybrid QAM or other type of modulation), a bandwidth of W is occupied to transmit a bit rate of R. If N users wish to transmit simultaneously in the same manner and at the same bit rate R, the bandwidth occupied is Nxc3x97W. This is obviously true for FDMA (Frequency Division Multiple Access) encoding. It is also true for TDMA (Time Division Multiple Access) transmission.
In order to describe the basic CDMA technique in more detail, and in a comparable manner, a spectrum spreading factor N is also chosen. N can take any value and a power of 2 is often chosen. In CDMA encoding, a binary symbol (+1 or xe2x88x921) to be transmitted is encoded by N bits (+1 or xe2x88x921) of an encoding sequence. Thus encoding is similar to multiplication. The symbols thus encoded N times (N xe2x80x9cchipsxe2x80x9d) are transmitted during a symbol period. This also occupies a bandwidth of Nxc3x97W.
For spectrum spreading resulting from this multiplication, N (and only N) orthogonal sequences can be allocated to N users. In principle, because they are orthogonal, the sequences do not interfere with each other. If there are more than N users the orthogonal CDMA technique cannot be used. In particular, it follows from the orthogonal nature of the encoding sequences that the content of a message of one user does not interfere at all with the content of a message of another user. This is denoted I/S=0, where I represents the interfering energy due to messages different from a message to be transmitted and S represents the energy of the message to be transmitted.
However, in CDMA encoding, it is possible to encode the symbols of messages to be transmitted using random or pseudo-random sequences of N bits rather than orthogonal sequences. In this case, very long random or pseudo-random series of bits (+1 xe2x88x921) are preferably used (several million bits, for example). Successive and contiguous sections of N sampled bits from the series constitute the encoding sequences. With this kind of random or pseudo-random encoding, a symbol to be transmitted is encoded using a sequence of the above kind, a subsequent symbol is encoded using a subsequent sequence (a subsequent section of N bits from the series of random or pseudo-random bits), and so on.
Although there is no interference with orthogonal sequences of N bits, it can be shown that, because the random or pseudo-random sequences are not orthogonal (or at least no systematically orthogonal), the interference in each message from the channel is of the type K/N where K is the number of users, the number of messages to be transmitted. Because transmission quality is progressively degraded as a function of the number of messages to be transmitted simultaneously, PN CDMA transmission leads to a limit very much lower than N on the number of simultaneous messages that is feasible (for an encoding sequence of N random or pseudo-random bits), because if that number became equal to N, the ratio K/N would become equal to 1: the interference noise would have the same power level as the wanted signal.
The demand for transmission bit rates is ever increasing and as a result the problem of transmission channel saturation is routinely encountered.
The invention remedies the problem of this limit of messages if the encoding sequences are N bits long. or N first messages the invention uses orthogonal encoding sequences of N bits. This is easy if the number of messages is less than or equal to N. For additional messages, random or pseudo-random (PN) encoding sequences are used that interfere with each other and with the orthogonal sequences. The reception process preferably involves a number of steps. The message signals encoded using orthogonal sequences and those using PN sequences are separated by separate decoding. The proposed multiple access technique can thus accommodate N users without any mutual interference, or a greater number of users at the cost of a slightly degraded signal to noise ratio. However, as shown in the remainder of the description, the effects of such noise can be neutralized by channel encoding methods already in use which provide transmission redundancy. It is then as if M users could together use a channel theoretically limited to N users, when M is greater than N (in one example, M can be in the order of N plus 25% of N).
The invention therefore consists in a spread spectrum transmission method, in particular of the CDMA type, for transmitting a plurality of messages each containing a series of symbols, in which method:
each symbol to be transmitted is encoded using a sequence of N bits to obtain for each symbol a sequence of N contributions known as chips,
the chips of the various messages are combined to obtain a transmitted signal, and
the transmitted signal is received, characterized in that:
if N symbols at most are transmitted simultaneously, orthogonal sequences are chosen for the sequences of bits used,
if M symbols are transmitted simultaneously, where M is greater than N, N orthogonal sequences of N bits are chosen for N symbols and random or pseudo-random sequences of N bits are chosen for the remaining M-N symbols,
a sequence of N bits being random or pseudo-random if for subsequent symbols of the same message to be transmitted the sequences of N bits are reproduced only with a period greater than N.
For reception, the method of the invention is characterized in that the following steps are executed:
a) the transmitted signal received is decoded using the N orthogonal sequences and N quantified decoded signals are obtained, the N quantified decoded signals are detected and N estimates of the symbols of the N messages are obtained,
b) the transmitted signal received is decoded using the M-N random or pseudo-random additional sequences and M-N quantified decoded additional signals are obtained,
c) interference synthesized from the N estimates of the symbols of the N messages is subtracted from each of the M-N quantified decoded additional signals, the results of the subtractions are detected and M-N additional estimates of the symbols of the M-N messages are obtained.